The essential thing about LADA is that it is, like T1, autoimmune. Beyond that it isn't that well defined and studied . (and as studies often use slightly different definitions they can't all be compared)
This article from a US professional journal is reasonably comprehensive and has some charts of characterististics
http://www.aafp.org/afp/2010/0401/p843.html
Beyond that type 1.5 doesn't really mean the same thing to all people.
Sometimes MODY is called 1.5. That is a group of diabetic types caused by a single change in DNA (so is normally associated with a family history)
http://www.diabetesgenes.org/content/maturity-onset-diabetes-young
And then T2 doesn't have a clear definition either. It's all those people who don't fit into the other boxes. Most undoubtedly are overweight at diagnosis but not all.
You only have to read on here that there are a number of people who aren't and it's not clear why. Here's what I think could be possible reasons (from reading )
They could be insulin resistant: overfull fat cells will occur at a lower weight in people who have few or small numbers of fat cells.
Their beta cells aren't producing enough insulin (but no autoimmune attack killing off beta cells)
The signal to produce insulin may not work well .
The body doesn't respond to rising glucose levels at the normal level
All are possible reasons for higher glucose levels but can't necessarily be tested They will end up being called T2 because there is no other box at present to stick them in.